This invention relates to avionics power supplies, and, more particularly, to a dual function power conversion module for isolating, converting and regulating voltage supplied by the prime power source of the aircraft to the DC levels necessary to power avionics cryptographic processors and for providing battery back-up to support memory retention in the cryptographic device when prime power is lost or removed.
Future combined military communication, navigation and Identification (“CNI”) Avionics systems will include secure processing to ensure that vital communications and data transmission are secure. Security requires that secure processing devices, such as cryptographic devices, use isolated power supplies to electrically isolate the electronic circuits of the device from other electronics circuits of the CNI system. The electrical isolation prevents sensitive data (in unencrypted form) from being coupled unintentionally to and transmitted by other sections of the avionics system through which the sensitive data could be intercepted by electronic eavesdroppers.
The encryption devices of the CNI avionics system make use of special data encryption algorithms, including encryption “keys”. When entered, the encryption algorithms are stored in semiconductor memory associated with the secure processor device, avoiding the necessity to re-enter the key each time encryption is used. To preserve the encryption algorithms, the foregoing semiconductor memory must be continuously supplied with DC power. The requirement for continuous power requires a source of “keep alive” power to the system, back-up power that ensures that the memory receives power even when prime power to the CNI system, the main power generator of the aircraft, fails or is shut down.
As example, the prime or main power of the aircraft is typically extinguished when the aircraft engine is shut down while the aircraft is parked on the tarmac for refueling and/or other maintenance. CNI systems must therefore support an isolated power source, provide backup power for the memory of the secure processor when prime power is not available to preserve the encryption keys and permit periodic maintenance to be performed, including replacement of the back-up battery.
Generally speaking, power conversion and battery back up systems of one sort or another are known. Ostensibly such known systems should be available to serve the general functions described in the foregoing paragraphs. However, application in aircraft systems imposes special constraints. Hardware of the avionics system is designed to minimize weight and space requirements and maintenance. To ease the maintenance procedure, avionics systems preferably employ the Line Replaceable Module (“LRM”) concept of product packaging. In application of the foregoing concept functional portions of the CNI avionics hardware are packaged in individual modules that “plug-in” to a backplane type interconnection circuit. That backplane electrically unites the individual modules into an integrated system. With the foregoing, replacement of individual modules may be accomplished easily by unplugging the respective module and plugging in a substitute. Thus, the “downtime” of the aircraft for maintenance is kept to a minimum.
One design approach, thus, is to include a back up battery for the earlier described purpose and a power supply that does not emit spurious frequencies (e.g. a non-spurious frequency emitting power supply), sometimes referred to as a TEMPEST power supply, in individual LRMs. For such a design, two separate LRMs are required to provide the functions of battery backup and the non-spurious frequency emitting power supply. Additional interconnection wiring between the foregoing LRMs and other LRMs of the avionics system may be accomplished in great part through the backplane. In such a system design prior to the present invention, the back-up battery alone required about 90 cubic inches of space, was three and one-half pounds in weight and, due to the limited life span of two to three years available with existing batteries, six thousand dollars in recurring maintenance costs. The power converter required about 23 cubic inches and adds another one and six-tenths pounds in weight.
As an advantage the present invention accomplishes the same function using a single LRM, thereby reducing both the weight and volume of the components of the avionics system. As a further advantage, the consolidation of both the described functions within a single LRM permits installation within an electrically isolated section of the avionics rack (backplane). That isolated section totally isolates the power, return and signal lines within the secure processing section of the avionics system from the other electronic circuits of the CNI system.
Isolation of spurious frequency emissions as might propagate into or through the power supply for the avionics system has been accomplished through inclusion of passive RF filters and through DC isolation of the component parts of the avionics system. Although to date such methods have been acceptable, some RF leakage was possible. As an advantage, the present invention not only incorporates passive RF filters, but takes a proactive approach to suppressing such emanations.
Accordingly, an object of the invention is to provide a power system for a CNI avionics system that is small in size and low in weight.
An additional object of the invention is reduce the weight and size requirements of the power supply and battery back up circuits and enhance the electrical isolation of those circuits in a CNI avionics system.
A further object of the invention is to actively prevent propagation of spurious RF frequencies through the power system for an avionics system and enhance isolation of the power system.
A still further object of the invention is to integrate an isolated power conversion system, a battery, and a battery charger driven by the power conversion system within a single housing or LRM.
And a final object of the invention is to increase the reliability of and reduce the cost of maintenance of CNI avionics systems.